1. Field of the Invention
Generally, embodiments of the present invention relate to a plunger lift system for artificially lifting fluid. More specifically, embodiments of the present invention relate to a lubricator for a plunger lift system used to lift fluid from a well.
2. Description of the Related Art
To obtain hydrocarbon fluid from an earth formation, a wellbore is drilled into the earth to intersect an area of interest or hydrocarbon-bearing reservoir within a formation. The wellbore may then be “completed” by inserting casing within the wellbore and setting the casing therein using cement. In the alternative, the wellbore may remain uncased (an “open hole wellbore”), or may become only partially cased. Regardless of the form of the wellbore, production tubing is typically run into the wellbore (within the casing when the well is at least partially cased) primarily to convey production fluid (e.g., hydrocarbon fluid, which may also include water) from the reservoir within the wellbore to the surface of the wellbore.
Often, pressure within the wellbore is insufficient to cause the production fluid to naturally rise through the production tubing to the surface of the wellbore. Thus, to carry the production fluid from the reservoir within the wellbore to the surface of the wellbore, artificial lift means is sometimes necessary. Some wells are equipped with a plunger lift system to artificially lift production fluid to the surface of the wellbore.
A plunger lift system generally includes a piston, often termed a “plunger,” which cyclically travels the length of the production tubing. The plunger essentially acts as a free piston to provide a mechanical interface between lifted gas from the formation disposed below the plunger and the produced fluid disposed above the plunger, thus increasing the lifting efficiency of the well.
FIG. 1 illustrates a typical plunger lift system within a wellbore 40 formed in an earth formation 85 to intersect a reservoir 80. The formation 85 includes one or more perforations 90 therein for allowing flow of production fluid from the reservoir 80 into the wellbore 40. The typical plunger lift system installation includes a tubular 45, which is usually production tubing, disposed within the wellbore 40.
Disposed proximate a lower end and within a longitudinal bore running through the production tubing 45 is a bottomhole assembly including upper and lower tubing stops 65, 75 having a standing valve 70 therebetween. A lower bumper spring 60 is located above the upper tubing stop 65, and a plunger 55 for lifting well fluid is disposed above the lower bumper spring 60. The lower bumper spring 60 and the tubing stop 65 provide a shock absorber at the lower end of the production tubing 45 to cushion the plunger 55 at the end of its down-stroke.
FIG. 1 shows the standing valve 70 as a separate component from the lower tubing stop 65 and the lower bumper spring 60. In some configurations of bottomhole assemblies, the standing valve 70, lower tubing stop 65, and lower bumper spring 60 all constitute one assembly. In other configurations, two or more of the standing valve 70, lower tubing stop 65, and lower bumper spring 60 may be combined with one another to constitute a portion of the bottomhole assembly. In either case, the lower bumper spring 60 may have a ball and seat integral therewith.
A fluid load 50, which is generally a liquid load of production fluid and/or water, is shown in FIG. 1 being lifted upward toward a surface 10 of the wellbore 40 by the plunger 55. Once the fluid is lifted by the plunger 55, it flows upward through the production tubing 45 until it reaches surface equipment. The surface equipment includes a lubricator 100 for absorbing the shock of force exerted by the upwardly-moving plunger 55 at the end of the plunger's up-stroke. In its cycle, the plunger 55 runs within the bore of the production tubing 45 for the full length of the production tubing 45 between the lower bumper spring 60 and the lubricator 100.
The lubricator 100 is installed on top of a master valve 35 disposed at the surface 10. A first fluid flow outlet 110 and a second fluid flow outlet 120 provide exit paths for the liquid load 50 which may be selectively opened and closed by a plug valve 5 and a valve 15, respectively. Both fluid flow outlets 110, 120 merge into a single flow line which a motor valve 30 is used to open and close. A pressure controller 20 operates the motor valve 30 to form a product 25.
FIG. 2 shows a typical lubricator 100 provided in the plunger lift system having an upper end 101 and a lower end 102. The lower end 102 is connected to the master valve 35 (see FIG. 1).
The lubricator 100 includes a tubular body having a first tubular section 125, usually termed a “spring housing,” connected to a second tubular section 145. O-rings 165 are provided at the connection point between the tubular sections 125, 145 to prevent fluid communication between a bore 115 of the lubricator 100 and the atmosphere (see FIG. 1). A cap 130 is connected to an upper end of the spring housing 125. The top of the cap 130, and therefore the upper end 101 of the lubricator 100, is usually flat-shaped, as shown.
The first and second flow outlets 110, 120 and a catcher assembly 140 extend from the tubular body. The catcher assembly 140 retains the plunger 55 to facilitate inspection of the plunger 55. Also extending from the tubular body are handles 135 to permit lifting of the lubricator 100.
At an upper portion of the tubular body, the lubricator 100 includes an upper bumper spring 103 within the bore 115 to attempt to absorb the shock or kinetic energy of the plunger 55 at the end of its up-stroke. A striker assembly 105 (also termed “bumper plate” or “striking pad”), which is disposed within the bore 115 directly below the upper bumper spring 103, provides the solid contact point for the plunger 55. The striker assembly 105 includes an opening 104 which allows fluid communication between the portions of the bore 115 above and below the striker assembly 105.
In operation, the plunger 55 cycles between the lubricator 100 (specifically the striker assembly 105 and upper bumper spring 103) and the bottomhole assembly (specifically the lower bumper spring 60 and the upper tubing stop 65). The bumper springs 103, 60 attempt to absorb the shock or kinetic energy of the plunger 55 at the ends of the up-stroke and down-stroke, respectively, of the plunger lifting cycle.
Using the bumper spring within the lubricator to absorb the shock of the plunger on its up-stroke is problematic because of additional safety hazards which occur with use of the lubricator as well as because of decreased profitability of the well with use of the lubricator. The force of impact of the plunger against the spring often causes the bumper spring to fail, break, or become otherwise damaged. Damage to the spring may require replacement of the spring, decreasing the profits of the well because of down-time during spring replacement. Additionally, damage to the spring may decrease the shock absorption ability of the spring, eventually causing the plunger to blow out the cap and exit the lubricator into the atmosphere. Blowing off the cap from the lubricator creates a safety hazard and usually causes damage to the lubricator, also decreasing the profitability of the well due to down-time to replace or repair the lubricator. Finally, damage to the spring may cause damage to the plunger upon its impact with the striker assembly due to ineffective or non-existent cushioning of the plunger because the damaged spring is dysfunctional or non-functional, ultimately increasing the cost of the well not only because of down-time which occurs to replace or repair the plunger, but also because of the additional cost of replacement parts, specifically the plunger.
Moreover, use of the lubricator having the bumper spring is problematic because damage or failure of the bumper spring, plunger, or other internal components is not detectable using this spring-based lubricator without stopping the plunger lift operation (down-time) and removing the internal components from the lubricator for inspection. Blowout of the plunger from the lubricator upon damage or failure of the internal components is not preventable because of the inability to determine the condition of the internal components during operation of the lubricator (as viewing the internal components is prevented by the presence of the tubular body).
Therefore, there is a need for a lubricator having an improved ability to cushion the plunger at or near the end of its up-stroke. There is a further need for a lubricator which is capable of absorbing the kinetic energy of the plunger at the end of the up-stroke without damaging portions of the lubricator. Furthermore, there is a need for a lubricator which allows monitoring of the plunger energy-absorbing ability of the lubricator in real time during operation of the plunger lift system.